Originally Posted by Lothlorian
We are restoring our 1960
airstream back to the way it was originally. My 12 volt
incadescent lights keep blowing after a very short time. I checked the voltage and I keep getting 13.6 volts. What could be the problem?
Incandescent light bulbs are extremely voltage sensitive. It has been this way since the days of Edison, and in olden times there were both 110 volt and 120 volt bulbs for sale in stores so that people could adjust for the voltage they experienced. Incandescent bulbs being resistive the power at the filament goes by the square of the voltage so applying 13.6 volts to a bulb designed for 12 volts applies 1.28x the rated power. As a result the lights burn out quickly since the filament temperature is much higher than it should be.
In the 1950s and 1960s when trailers that used the 12v Edison-base screw-in light bulbs were built, there were no converters. The battery was charged (to the extent the term can be said to apply) only by the charge line from the tow vehicle. Therefore it was not being charged while the lights were in use. The battery would deliver a float voltage of typically 12.7 volts. Since there is some voltage drop in the wiring the voltage at the lamp itself was typically between 12.0 and 12.5 volts, within the range for which the lamps were designed.
I assume your restoration includes a modern converter. This is keeping the system voltage closer to 13.6 volts in order to charge the battery and it is this change that is causing your lamps to fail early.
Since screw-in (Edison base) lamps aren't available in 13.6 volts, your choices are limited.
You could avoid using the converter and the lights at the same time.
You could use a dropping resistor in line with the lamps. For 25 watt lamps an 0.5 ohm, 2 watt resistor would be about right. You may be able to improvise something with ignition ballast resistors, which are in roughly the right ohm and power range. For example you could hook up 3 1.5 ohm ballast resistors in parallel. For 50 watt lamps you would want an 0.25 ohm, 5 watt resistor.
You may be able to lower the output voltage of your converter although this will mean that the battery will take much longer to charge.
Hope this helps. I like the old dual-volt setups and it's good to see some of them live on.